1. Field of the Invention
The present invention relates to a spark-ignition gasoline engine.
2. Description of the Related Art
A spark-ignition gasoline engine is regarded as following an Otto Cycle in theory, wherein its theoretical thermal efficiency ηth is expressed by the following Formula, as disclosed, for example, in “Internal Combustion Engine Fundamentals” (Document D1) authored by John B. Heywood:ηth=1−(1/εκ−1)  (1)                wherein ε is a compression ratio, and κ is a specific heat ratio.        
As is clear from Formula (1), the theoretical thermal efficiency (i.e., indicated, net thermal efficiency) of the spark-ignition gasoline engine is improved up to a certain level as the compression ratio is set at a higher value. In this relation, the Document D1 reports a research on changes in theoretical thermal efficiency at various compression ratios (8≦ε≦20) under the conditions that a spark-ignition gasoline engine is operated at wide open throttle (WOT) and 2000 rpm. The report says that each of the theoretical thermal efficiency and a mean effective pressure (MEP) goes up in proportion to an increase in compression ratio up to around 17, and then remains on the same level despite a further increase in compression ratio.
Based on the above research result, great efforts have been made for practical realization of an engine with a higher compression ratio (i.e., high-compression engine).
In reality, a high-compression spark-ignition engine inevitably involves lowering of engine power due to engine knock occurring in a high-load operation zone including a wide open throttle region.
As conventional measures against this problem, there has been widely known an ignition retarding control of retarding an ignition timing. However, it has been considered that a technique of avoiding knock based on the ignition retarding control causes excessive lowering of engine power in a high-load operation zone and critical deterioration in merchantability.
FIG. 1 is a graph showing one example of the ignition retarding control in a high-load operation zone.
As shown in FIG. 1, for example, in a usual compression ratio (ε=11) which is widely employed in conventional engines, no knock occurs when the ignition timing is set at 4 degrees (CA) before a top dead center of a compression stroke (hereinafter referred to as “compression TDC”). By contrast, in a high compression ratio (ε=13), a knock occurs even when the ignition timing is set at 4 degrees before a compression TDC. Therefore, it has been considered that a high compression ratio has to be employed in combination with a greater amount of ignition timing retard. This led to one conclusion that a decrease in engine power due to ignition timing retard required for preventing the occurrence of knock at an increased compression ratio of about 13 goes beyond an increase in engine power provided by the increased compression ratio. Thus, in consideration of lowering in engine power due to ignition timing retard, conventional high-compression engines have been designed to set an upper limit of compression ratio at 12 for a high-load operation zone including a wide open throttle region, and avoid using a higher compression ratio than the upper limit in the high-load operation zone.
As to a high-load operation zone including a wide open throttle region, there has been known a technique of reducing an effective compression ratio using a so-called “Atkinson Cycle” or “Miller Cycle”. However, if the effective compression ratio is reduced by changing an intake-valve closing timing during a high-load operation, an in-cylinder pressure is lowered in an intake stroke due to loss of fresh air to cause deterioration in charging efficiency and lowering in engine power.
With a view to avoiding this problem, there has also been known a technique of reducing a geometrical compression ratio of an engine in a high-load operation zone including a wide open throttle region. For example, JP 2005-076579A (Document D2) and JP 2005-146991A (Document D3) disclose a technique of changing a geometrical compression ratio depending on engine operation states by use of a variable compression ratio mechanism provided in an engine.
The technique disclosed in the Documents D2, D3 is designed to reduce a compression ratio in a wide open throttle region so as to avoid the occurrence of knock. Therefore, an approach to high compression ratio in spark-ignition gasoline engines has been obliged to choice between only two techniques: one achieved at the sacrifice of engine power; and the other achieved at the sacrifice of cost.
Moreover, the use of a mechanism for changing a geometrical compression ratio, as disclosed in the Documents D2, D3, leads to structural complexity of an engine and increase in cost.
In view of the above problems, it is an object of the present invention to provide a spark-ignition gasoline engine having both a low-cost performance and a high engine-power performance even in a high-load operation zone (particularly wide open throttle region) in a low speed range.